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I want to drive several irrigation solenoid valves with solid state relays. The solenoid in question is a Hunter model 606800 24V AC device. Its specifications state 370mA inrush current and 210mA holding current at 24VAC 50Hz.

I've looked at many SSR spec sheets but on none of them can I see any mention of any issue when driving an inductive load. For example this SSR: https://www.farnell.com/datasheets/2144238.pdf

So will such an SSR drive an inductive load like a solenoid?

Edit: would a symmetrical Transient Voltage Suppressor be a good option? Across the solenoid or the SSR?

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2 Answers 2

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So will such an SSR drive an inductive load like a solenoid?

Because the solenoid is driven with AC, a simple flyback diode cannot be used. So, to prevent a back-emf (from the solenoid) destroying the SSR (rated for 60 volt peak maximum) you will need to use a snubber circuit (R and C in series across the solenoid).

However, to design the snubber circuit you will need to know the inductance of the solenoid. Once you have this value you can easily simulate the circuit to determine the snubber circuit required.

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  • \$\begingroup\$ Alternately, a bidirectional TVS diode, rated somewhat above the nominal-maximum peak input voltage (36V or so?), and below the SSR rating, will absorb whatever kick occurs. This can also be used in combination with the RC for faster damping. \$\endgroup\$
    – Tim Williams
    Commented Aug 3, 2023 at 13:54
  • \$\begingroup\$ @TimWilliams would you put the TVS across the solenoid or across the SSR? \$\endgroup\$
    – jox956
    Commented Aug 4, 2023 at 0:08
  • \$\begingroup\$ @jox956 SSR, to be clear. \$\endgroup\$
    – Tim Williams
    Commented Aug 4, 2023 at 0:13
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A zero-crossing SSR may help somewhat. It switches when the voltage is zero and the current is lower (not zero though due to the lag of current behind voltage for an inductor). I would still include a snubber, but one with generic component values may be sufficient.

Alternately, you can just run the solenoids on DC with a flyback diode. They can run on that fine. The disadvantages are that the solenoids will waste more energy while open. When run on AC, the changing inductance of the solenoid causes it to draw more power in the closed position. This effectively gives it a higher current "kick" open. You don't get that with DC. Also you will need to determine a good dc operating voltage. It will be a bit lower than the AC voltage.

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  • \$\begingroup\$ Zero crossing of voltage is probably about the worst place for current being close to maximum. \$\endgroup\$
    – Andy aka
    Commented Aug 3, 2023 at 13:20
  • \$\begingroup\$ @Andyaka At 50Hz I doubt it's anywhere close to a purely inductive load. \$\endgroup\$
    – Drew
    Commented Aug 3, 2023 at 13:26
  • \$\begingroup\$ I think a triac output SSR will naturally disengage when the gate signal is removed and the anode current falls to zero so, maybe that's what you meant? They will naturally go-open when anode current is close to zero. \$\endgroup\$
    – Andy aka
    Commented Aug 3, 2023 at 13:37
  • \$\begingroup\$ @Andyaka I agree that would be better. But afik triac output SSRs aren't common. I wonder what the phase difference actually is at 50Hz. If I had a sprinkler valve lying around I would test it. \$\endgroup\$
    – Drew
    Commented Aug 3, 2023 at 14:06
  • \$\begingroup\$ I think they might be more common than you think. I don't know why you have received a downvote btw. \$\endgroup\$
    – Andy aka
    Commented Aug 3, 2023 at 15:06

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